When an air-conditioning apparatus such as a multi-air-conditioning apparatus for a building performs a heating operation under low outside air temperature, a discharge temperature of a compressor is excessively high, and hence a frequency of the compressor cannot be increased, with the result that required heating capacity cannot be exerted. Further, when a refrigerant such as R32 is used, the discharge temperature of the compressor is excessively high even during a cooling operation under low outside air temperature. Therefore, it is necessary to lower the discharge temperature of the compressor so that an amount of heat corresponding to a load is supplied. To lower the discharge temperature of the compressor, known techniques provide a circuit for performing liquid injection to the middle of the compressor from a high-pressure liquid pipe of a refrigeration cycle, or an air-conditioning apparatus capable of controlling the discharge temperature to a set temperature irrespective of an operation state (for example, Patent Literature 1).
Further, known techniques provide an air-conditioning apparatus capable of injecting high-pressure liquid refrigerant of the refrigeration cycle to a suction side of the compressor in both of the cooling operation and the heating operation (for example, Patent Literature 2).
Still further, known techniques provide an air-conditioning apparatus including a subcooling heat exchanger on an outlet side of a condenser, and being configured to control a flow rate of refrigerant to flow through the subcooling heat exchanger, thereby controlling the discharge temperature of the compressor (for example, Patent Literature 3).